Intact eggs can become contaminated with Salmonella as a result of infections of the reproductive tissues of the laying hens but also by penetration through the shell. In this paper, the penetration of Salmonella through the shell of hen eggs is reviewed. A description is given of the advantages and disadvantages of the various methods used to study bacterial penetration through the shell and membranes. The possibility of Salmonella contamination of the shell after lay is included because shell contamination is the first requisite for penetration. Various factors affect the probability of bacterial penetration. Both the intrinsic and extrinsic factors are highlighted.

Following oviposition, the shell acquires contamination from all surfaces with which it makes contact, and the extent of contamination is directly related to the cleanliness of these surfaces (Board and Tranter, 1995). Limited data are available on the prevalence of shell contamination with Salmonella. In a hen house which Salmonella was isolated from 72% of the environmental samples, 7.8% of the shells were contaminated (Jones et al., 1995). The highest percentage of contaminated eggs and the fastest penetration rates were
observed when a positive temperature differential (egg warmer than the environment) was created between the eggs and the Salmonella culture used for dipping the eggs. When the egg is warmer, the subsequent cooling caused its contents to contract. The resulting
negative pressure aids drawing the bacteria through the pores (Bruce and Drysdale, 1994). Cooling eggs soon after laying at 4°C for 15 min before immersing them in a suspension helps prevent this.

Bottom line:Eggshells can be penetrated by various bacteria, among which is Salmonella Enteritidis. This serotype is linked with human salmonellosis cases caused by consumption of eggs and egg products. These findings supported that the cleaner the surface upon which the egg is being laid, and the methods used to wash eggs, were important to reducing Salmonella. Eggs that were already clean when gathered, were best left unwashed do to the protective coating already on the egg.